1 Mollusk carbonate thermal behaviour and its implications in 2 understanding prehistoric fire events in shell middens 3 Stefania Milano1,2*, Susanne Lindauer3, Amy L. Prendergast4, Evan A. Hill5, Chris O. Hunt6, Graeme 4 Barker7, Bernd R. Schöne2 5 6 7 1 Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, 04103 8 Leipzig, Germany 9 2 Institute of Geosciences, University of Mainz, Joh.-J.-Becherweg 21, 55128 Mainz, Germany 10 3Curt-Engelhorn-Zentrum Archaeometry gGmbH, Klaus-Tschira-Archaeometry-Centre, C4, 8, 68159 Mannheim, 11 Germany. 12 4 School of Geography, University of Melbourne, 221 Bouverie St, Carlton, 3053, VIC, Australia 13 5 School of Natural and Built Environment, Queens University Belfast, Elmwood Ave, Belfast BT9 6AY, UK 14 6 School of Natural Sciences and Psychology, Liverpool John Moores University, Byrom Street, Liverpool, 15 United Kingdom 16 7 McDonald Institute for Archaeological Research, University of Cambridge, Downing St, Cambridge CB2 3ER, 17 UK 18 19 20 21 * Corresponding author. Email: [email protected] 22 23 24 25 Keywords: Carbonate phase transformation; Haua Fteah; Shell microstructure; Raman spectroscopy; 26 Thermal-induced diagenesis; Pyrotechnology 27 28 29 Abstract 30 Archaeological shell middens are particularly important for reconstructing prehistoric human 31 subsistence strategies. However, very little is known about shellfish processing, especially when related 1 32 to the use of fire for dietary and disposal purposes. To shed light on prehistoric food processing 33 techniques, an experimental study was undertaken on modern gastropod shells (Phorcus lineatus). The 34 shells were exposed to high temperatures (200-700 °C) to investigate subsequent mineralogy and macro- 35 and microstructural changes. Afterwards, the three-pronged approach was applied to archaeological 36 shells from Haua Fteah cave, Libya (Phorcus turbinatus) and from shell midden sites in the United Arab 37 Emirates (Anadara uropigimelana and Terebralia palustris) to determine exposure temperatures. 38 Results indicated that shells from the Haua Fteah were exposed to high temperatures (600 - 700 °C) 39 during the Mesolithic period (c. 12.7 - 9 ka), whereas specimens from the Neolithic period (c. 8.5 - 5.4 40 ka) were mainly exposed to lower temperatures (300 - 500 °C). The thermally-induced changes in A. 41 uropigimelana and T. palustris shells from the South East Arabian archaeological sites were similar to 42 those seen in Phorcus spp. suggesting a broad applicability of the experimental results at an interspecific 14 43 level. Although heat significantly altered the appearance and mineralogy of the shells, CAMS ages 44 obtained on burnt shells fit within the expected age ranges for their associated archaeological contexts, 45 indicating that robust radiocarbon ages may still be obtained from burnt shells. Our study indicates that 46 the combination of microstructural and mineralogical observations can provide important information to 47 infer shellfish processing strategies in prehistoric cultures and their change through time. 48 49 50 51 1. Introduction 52 Shells grow incrementally throughout the lifetime of mollusks and function as protection and support 53 structures. Shells also serve as excellent palaeoenvironmental archives (i.e. Jones, 1983; Schöne et al., 2 54 2004; Butler et al., 2013), because they faithfully record the physical and chemical conditions of their 55 ambient environment and temporal changes to these. Such information is stored in the form of 56 geochemical and structural properties (Epstein, 1953; Goodwin et al., 2001; Schöne, 2008). 57 Sclerochronology is the research field that studies the temporal context of shell chemical composition 58 (i.e. stable isotopes and trace elements) and physical accretionary patterns to produce extremely highly 59 resolved palaeoenvironmental reconstructions (Schöne et al., 2005; Miyaji et al., 2007; Milano et al., 18 60 2017; Oschmann, 2009). For example, shell oxygen isotope content (δ Oshell) is routinely used as 61 paleothermometer (Schöne et al., 2005; Ferguson et al., 2011; Prendergast et al., 2013; Prendergast and 62 Schöne, 2017). 63 64 A rapidly growing interest in the research field of sclerochronology supports the spread of its 65 methodologies and approaches to different disciplines such as archaeology and environmental 66 biomonitoring (Mannino and Thomas, 2002; Andrus, 2011; Steinhardt et al., 2016; Schöne and Krause, 67 2016). The analysis of mollusk shell material is especially relevant within the framework of prehistoric 68 archaeology. Shellfish have been an important dietary component since the emergence of anatomically 69 modern humans (~ 300 kyr ago; de Lumley, 1966), due to their easy accessibility, reliable availability 70 throughout the year and source of proteins and micronutrients essential for physical development 71 (Erlandson, 2001; Broadhurst et al., 2002; Marean et al., 2007; Fa, 2008). 72 73 The application of sclerochronology substantially broadens the potential use of mollusk shells in 74 archaeology. Besides being suitable palaeoenvironmental archives, the shells become a key tool for 18 75 understanding the response of human behaviour to climatic changes. For instance, δ Oshell is used to 76 constrain the season of mollusk collection (Mannino et al., 2007; Burchell et al., 2013; Prendergast et 3 77 al., 2016). In turn, seasonality provides information on the seasonal mobility of hunter-gatherer societies 78 and helps to identify whether sites were permanently or ephemerally occupied (Shackleton, 1973; 79 Mannino and Thomas, 2002; Eerkens et al., 2013). However, the processes related to mollusk 80 preparation, consumption and disposal are still largely unknown (Milano et al., 2016). In some cases, it 81 has been observed a modification of the shell structure by removal of the gastropod apex with stone tool, 82 thorn or canine tip to allow the mollusk to be sucked out (Girod, 2011). As for bivalves, the shells were 83 sometimes scarred or wholesale smashed (Hammond, 2014). Evidence of pyrotechnology associated 84 with mollusk shell middens suggests that a certain degree of heat exposure may have been involved in 85 some cases (Erlandson et al., 1999; Berna and Goldberg, 2008; Taylor et al., 2011). However, few 86 studies have addressed the reconstruction of such processes in the framework of prehistoric marine 87 resource exploitation (Andrus and Crowe, 2002; Aldeias et al., 2016; Milano et al., 2016) and this study 88 will be the first instance where this has been carried out. 89 90 The present study builds upon previous work by Milano et al. (2016) who found that significant 91 structural and chemical changes occurred in modern mollusk shells when they were heated at ≥ 300 °C 92 for 20 minutes or more. Here, we investigate the effects of a shorter heat exposure (5 minutes), since 93 shellfish are generally thought to be processed for short periods of time. To achieve this aim, modern 94 Phorcus lineatus are used as a calibration tool to understand the response to thermal treatments at the 95 macro-, microstructural and geochemical level. These findings are then applied to archaeological 96 specimens from the Haua Fteah, eastern Libya (Phorcus turbinatus) to test whether high temperatures 97 exposure can be detected in archaeological shells of the same genus using the three-pronged approach 98 developed in the experimental phase. Archaeological specimens from the United Arab Emirates 99 (Anadara uropigimelana and Terebralia palustris) are used to understand the efficacy of this three- 4 100 pronged approach on a broader scale to other mollusk species. Furthermore, given the importance of 101 shells for radiocarbon dating coastal sites and the abundance of burnt shells in middens (Douka et al., 102 2014; Lindauer et al., 2016), we present preliminary results on the influence of thermal alterations on 103 shell radiocarbon dating results. 104 105 2. Materials and methods 106 A total of 41 shells were analysed (Table 1). Fifteen live-collected shells of P. lineatus were used for the 107 experimental phase. The archaeological material consisted of eighteen specimens of P. turbinatus 108 selected from key occupation contexts in the Mesolithic and Neolithic layers from the Haua Fteah cave 109 in Libya, as well as six specimens of A. uropigimelana and two specimens of T. palustris from shell 110 midden sites near Kalba in the United Arab Emirates (Fig. 1). 111 112 2.1 Shell material: P. lineatus and P. turbinatus 113 Phorcus spp. are rocky shore gastropods living in the intertidal zone along the Mediterranean (P. 114 turbinatus: Menzies et al., 1992; Schembri et al., 2005; Mannino et al. 2008; Prendergast et al., 2013) 115 and Eastern North Atlantic coasts (P. lineatus: Kendall, 1987; Donald et al., 2012; Gutiérrez-Zugasti et 116 al., 2015). Both species are sensitive to seasonal environmental changes, specifically water temperature 117 fluctuations. Furthermore, they are extremely abundant in prehistoric sites of this region (Mannino and 118 Thomas, 2001; Colonese et al., 2011; Hunt et al. 2011; Bosch et al., 2015; Gutiérrez-Zugasti et al., 119 2015). Therefore, many previous studies have used their geochemical data for palaeoenvironmental 120 reconstructions (Mannino et al., 2003; Colonese et al., 2009; Prendergast et al., 2016). 121 5 122 In the present study, P. lineatus (previously known as Osilinus lineatus and Monodonta lineata) 123 was selected as a modern